In recent years, thermally regeneratable ion exchange resins have been developed which are suitable for partial desalinization, softening and desalinization and the like of liquids, particularly water. These resins include active groups having weak acid or weak alkaline function in the same ion exchange resin structure (matrix). They are pre-loaded with acid or alkali, so that the resin operates at a neutral pH during loading and regeneration. Depending on whether bivalent calcium and magnesium ions or monovalent sodium and potassium ions are to be absorbed, the degree of pre-loading varies. The attainable useful volume capacity and the elutriability of the ions absorbed depends on the degree of pre-loading, the so-called XO-value.
Thermal regeneration of such resins has the advantage that no strong, corrosive chemicals are required and therefore the tanks necessary for the loading and regeneration of the resins need not be equipped with expensive, acid- and alkali-resistant corrosion-protective linings. Since hot water is used for the regeneration, no salinization of the regenerated resin occurs. The impact of such methods on the environment is substantially less than that of the conventional ion exchange desalinization processes. If waste heat is available, this process can be made very economical.
In spite of the above potential advantages of the thermal method, the use of these ion exchangers has not achieved satisfactory efficiency levels due to various system-related defects.
In the plants known to date (August Journal of Chemistry 1966, pages 589 to 608, or "Desalinization" 1973, pages 217 to 237 and 269 to 285), the loading and regenerating takes place discontinuously, according to customary ion exchange techniques, in exchange vessels in a batch system which, after the resin is charged, the vessels are taken out of operation, regenerated with hot water and are then returned into the process.
The thermally regeneratable ion exchange resins have only very little usable volume capacity, (0.2 to 0.3 val/l.sub.A) wherein val represents one gram equivalent and l.sub.A represents one liter of exchange material whereby even at relatively low salt concentration in the raw water, a large quantity of ion exchange resin must be charged into the exchange vessels in order to obtain a sufficiently long working interval in relation to the regeneration time. This requirement, which is necessary for optimum operation, has adverse effects for several reasons:
The exchange vessels become very large and therefore, expensive. The large vessel volume of necessity requires large amounts of hot water for regeneration, as prior to each regeneration step, the cold water charge of the exchange vessel must be drained out and replaced by hot water or the cold vessel must be warmed up by additional hot water before the quantity of hot water required for the regeneration is introduced. Consequently, the heat utilization during the regeneration is relatively poor and the heat losses are high. Because of the large vessel volume and the large vessel surface, the heat losses during the regeneration are also considerable, so that, together with the disadvantages described above, only regeneration temperatures of 60.degree. to 80.degree. C. can be achieved with the batch system. However, the higher the temperature, the better is the regeneration effect. Preferred temperatures are up to 90.degree. C.
Automating the system is very expensive.
It is characteristic of thermally regeneratable ion exchange resins that the water to be desalinated should contain as little calcium and magnesium as well as bicarbonate as possible. If water is to be treated which has a high content of these substances, it is necessary to include softening or decarbonizing as a pre-treatment stage. It is a disadvantage of the conventional batch processes that the ion exchangers for softening and decarbonizing which precede the desalinization process, must be regenerated separately with chemicals and that thereby the efficiency of the process with thermal regeneration is diminished.
It is a further characteristic of the thermally regeneratable resins that heavy metals absorbed therein are not removed by the thermal regeneration. Therefore, dirt and heavy metals should not be in the exchange resin before it is thermally regenerated. If these substances are nevertheless present, a special treatment of the resin must be performed at certain intervals, such as regeneration with chemicals. After this special treatment, however, a new adjustment of the XO-value is required, so that the resin can again be regenerated in the next operating cycle. It is a disadvantage of the batch method that the operating phase of the plant is interrupted for an extended period of time for the special treatment and the subsequent adjustment of the XO-value and no desalinization takes place.
It is a further disadvantage of the conventional batch method that, due to the discontinuous change between regeneration and operating condition and the different regeneration and operating times, the degree of possible heat recovery from the hot elutriate produced in the regeneration is substantially smaller than with the present quasi-continuous system. The heat of the hot elutriate produced in the regeneration could be utilized, for instance, for heating the raw water or the water which is used for the regeneration. Even with multi-line operation of a batch plant, heat utilization without expensive intermediate storage of the hot elutriate with corresponding heat losses is not possible, because the regeneration time of the system is only a fraction of the operating time of the columns, and therefore, heating of raw or regeneration water can take place only during the relatively short regeneration time. If such a plant is designed with only one line, heat utilization without intermediate storage of the hot elutriate is practically impossible, as the flow through the operating column is interrupted during the thermal regeneration.
The same above-mentioned difficulties arise also if thermally regeneratable and reactivatable adsorption media are used instead of thermally regeneratable ion exchange resins. In the description of the present invention, which relates to thermally regeneratable ion exchange resins as well as to thermally regeneratable adsorption media, the substance that is to be regenerated and transported is therefore called "matter" for the sake of simplicity.